Temperature measurement is important in many processes. A common method of temperature measurement uses thermocouple transducers that output an EMF in response to a temperature gradient across two dissimilar materials, typically metals. It is well known, however, that thermocouples degrade over time due to chemical and metallurgical changes in the composition of the materials.
Common thermocouples used in temperature measurement comprise two dissimilar thermoelements connected at one end in a hot junction with the other ends connected to the positive and negative leads of a voltmeter at a known reference temperature. The temperature measured at the hot end is a function of the EMF measured and the reference temperature. The thermocouple circuit is typically constructed from conductive metal wires, and the associated loop resistance of the thermocouple monitoring circuit is a measure of the electrical resistance due to the various connections, the resistivity of the wires, and the junction of the materials at the hot junction.
During the use of thermocouples, several forms of degradation take place in the thermocouple circuit including chemical, metallurgical, and mechanical changes in the materials and devices of the circuit. Chemical changes include oxidation of elements in the alloys of the individual thermoelements that, in effect, modify the alloy composition of the base material. The change in chemical composition is usually accompanied by a shift in the resistivity of the thermoelement. Diffusion of the elements may also cause changes in the chemical makeup of the thermoelements and be a source of further resistivity change.
The junction of the two thermoelements is particularly susceptible to chemical changes. The junction is most often the hottest portion of the circuit and is therefore exposed to the harshest conditions. The junction is also exposed to processes that may increase the likelihood of changes in the electrical properties. Welding, soldering, twisting, or crimping, for example, commonly forms the junction of a thermocouple. These joining methods apply a large amount of heat in the case of welding, introduce new materials in the case of soldering, or mechanically strain the materials in the case of twisting or crimping. In these examples the degradation of the junction may be evidenced by an accompanying shift (change) in the loop resistance of the measurement circuit.
Metallurgical changes such as grain growth may also contribute to resistivity changes in the thermoelements. Grain growth, re-crystallization, or annealing is usually accompanied by a change in the resistivity of the material. Finally, severe mechanical damage such as sharp bends, kinks, or indentations can cause a change in the geometry of the thermoelements and the temperature measurement. Mechanical damage and thermal cycling may also change the contact resistance in screw terminals, connectors, or plugs. In these instances the base resistivity of the material is unchanged but the overall loop resistance of the circuit is impacted.
In all of these cases a measurement of the loop resistance of the circuit may help identify degradation of the measuring circuit. The use of impedance measurements on thermocouple circuits have been employed in the past to detect problems in the circuit. In one case, a complex oscillating signal circuit is used to derive the impedance measurement from the thermocouple circuit.
While the prior art has employed the value of loop resistance in determining thermocouple health, accommodations for specific aspects of most practical thermocouple circuits have not been made. For example, in most applications the circuit comprises a portion of the sensor that is exposed to temperature variations along with some leadwire circuitry that is maintained near room temperature. Typically the heat affected region of the circuit is short compared to the total loop length, thus the resistance of the loop is a combination of a large contribution from the leadwire and a smaller contribution from the actual measuring section. Also, since most of the common thermoelement materials have a significant temperature coefficient of resistance, any change in measured temperature will affect the total loop resistance.
Without a method to isolate the changes in loop resistance due to degradation from the effects of temperature change, however, the true health of the thermoelements is difficult to determine with any significant accuracy. Accordingly, there is a need for more accurately detecting the degradation of the thermoelement materials of a temperature monitoring system prior to failure, and for determining the health of the thermocouple and the temperature monitoring system.